Various microelectronic circuits are formed by means of thick-film technology which includes the furnace firing of an ink or paste formulated from materials that include a property determinator which will make the printed material either conductive, resistive or dielectric in nature. The paste is formulated using metal powders, a glass frit powder, binders and vehicles and placed or deposited in a selected pattern onto a nonconductive substrate (e.g., ceramic porcelainized steel, glass) printed and then dried to volatilize vehicle constituents contained in the paste such that it can then be fired to sinter or fuse the constituents bonding the film to the substrate.
Precious-metal conductive paste materials are prepared using a noble metal or combination of noble metals such as platinum, palladium, silver, or gold to permit electrical flow with minimal opposition. Resistive materials are pastes which contain a variety of substances such as carbon, thallium, indium, ruthenium, and many others. Dielectrics are prepared by using pastes containing glasses, ceramics, glazes, and ferro-electric materials. Precious-metal conductors, dielectrics, and resistors are typically designed to be compatible with each other and are fired at temperatures between 700.degree.-1000.degree. C. in an air atmosphere.
The use of an oxidizing air atmosphere is ideal for removing carbon-based vehicles whose main purpose is to impart the proper rheological properties during screen printing. In an air-fireable system oxygen is readily available to oxidize any of the organic vehicles that vaporize during the thick-film firing cycle to carbon monoxide (CO), carbon dioxide (CO.sub.2), and water vapor (H.sub.2 O). Gaseous CO, CO.sub.2, and H.sub.2 O are easily swept from the furnace by the exiting air atmosphere. The presence of an oxidizing atmosphere also serves to maintain the proper physical environment within the thick film, so that the sintering and adhesion mechanisms can occur properly.
In the past, in manufacture of the thick-film conductors, noble metals such as gold, silver, platinum and palladium have been used for conductors. Because of their low cost and better physical properties, attempts have been made to substitute base metals for noble metals. Copper, because of cost and physical properties (solderability and conductivity) is an ideal candidate. Air, previously used as a firing atmosphere, must be replaced by a neutral atmosphere (e.g. nitrogen, argon, helium, or mixtures thereof), to fire copper thick-films, conductors, resistors and dielectrics. Although useful in preventing the copper from oxidizing, a neutral atmosphere - typically nitrogen, does not provide an oxidizing agent capable of removing the carbon-based vehicle in an efficient manner. In the absence of an oxidizing agent the vehicles pyrolyze as the parts are processed.
The addition of oxygen or oxidizers to the nitrogen furnace atmosphere has led to problems since the oxidizer tends to oxidize the base-metal copper creating an adverse change in electrical characteristics and solderability properties because of formation of oxide coatings thereon.
In conventional copper thick-film practice, firing is carried out in a belt furnace having an atmosphere which is basically inert, but which contains small amounts of oxidizing agents (typically oxygen or air) to react with the vehicle and oxidize the components of the vehicle that are made from carbon-based constituents. The atmosphere is moved through the furnace to sweep the reactive products from the furnace. In the prior art of copper thick-film processes, the carbon-based vehicles do not react efficiently with the small amount of oxygen added to the nitrogen atmosphere and tend to form free carbon in the form of soot which can deposit on heating elements, on walls in the furnace as well as on the substrates being treated. Free carbon can cause defects in the electrical component, especially if it deposits between alternate layers of a multi-layer structure. Carbon deposited on the walls of the furnace must periodically be removed, thus adding to the cost of the operation. Single layer thick-film electrical components containing deposited carbon must be cleaned adding another step and additional cost to the production of such devices.
Attempts have been made to solve the problem by means of controlling the atmosphere by limiting the oxygen content as shown in U.S. Pat. Nos. 3,726,006; 4,296,272; 4,311,730; 4,313,262 and 4,517,155. The foregoing patents all show the use of oxygen to oxidize the volatillzed constituents of the vehicle to remove them from the furnace or the use of oxygen to provide a resistor by oxidizing the material deposited on the substrate.
Other attempts to solve the problem by means of atmospheric control or control of the composition of the printing ink are shown in U.S. Pat. Nos. 4,409,261 and 4,122,232.
In addition to the use of oxygen additions to the preheat zone of a multi-zone furnace used to process thick-film electrical components and the altering of the paste formulations, other solutions have been proposed which center around modifying the furnace design to assist in removal of the volatillzed vehicle from the printing ink, processing the parts in air using sacrificial iron containers to getter excess oxygen and to increase atmosphere flow rates into the furnace. Tien-Shou Wo et al., in their article describe a process by which parts are fired in air using sacrificial iron containers to preferentially react with excess oxygen. The continued use of large gas flow rates to physically sweep the carbon level vehicles out of the furnace is evidence that the above attempts have not solved the basic problem.
Furnace modifications have been made in attempts to evacuate the vehicle from the preheat zone more effectively. By introducing a greater volume of inert gas and designing several vent stacks into the preheat zone, the vehicles are swept from this area only to redeposit on the cooler vent stacks. This depositional process continues thus making it necessary to clean the vent stacks frequently or have partially decomposed carbon-based material drip onto the parts in the furnace.